The subject matter described herein relates generally to controlling operation of electric power systems, and more specifically, to equipment and methods for dynamically braking power converters.
Generally, a wind turbine includes a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. At least some of the known wind turbines are physically nested together in a common geographical region to form a wind turbine farm. Variable speed operation of the wind turbine facilitates enhanced capture of energy when compared to a constant speed operation of the wind turbine. However, variable speed operation of the wind turbine produces electric power having varying voltage and/or frequency. More specifically, the frequency of the electric power generated by the variable speed wind turbine is proportional to the speed of rotation of the rotor. A power converter may be coupled between the wind turbine's electric generator and an electric utility grid. The power converter receives the electric power from the wind turbine generator and transmits electricity having a fixed voltage and frequency for further transmission to the utility grid via a transformer. The transformer may be coupled to a plurality of power converters associated with the wind turbine farm.
The wind turbine may not be able to operate through certain grid events occurring downstream of the transformer, since wind turbine control devices require a finite period of time to sense the event, and then make adjustments to wind turbine operation to take effect after detecting such grid event. Therefore, in the interim period, the wind turbine may sustain wear and/or damage due to certain grid events. Such grid events include electrical faults that, under certain circumstances, may induce grid voltage fluctuations that may include low voltage transients with voltage fluctuations that approach zero volts. At least some known protective devices and systems facilitate continued operation during certain grid events. For example, for grid transients such as short circuits, a low, or zero voltage condition on the grid may occur. Under such conditions, such known protective devices and systems define a low and/or a zero voltage ride through (LVRT and ZVRT, respectively) capability. Such LVRT/ZVRT capabilities facilitate operation of the power converters of individual wind turbines and wind turbine farms to transmit reactive power into the utility grid. Such injection of reactive power into the grid facilitates stabilizing the grid voltage while grid isolation devices external to the wind farm, such as automated reclosers, will open and reclose to clear the fault while the LVRT/ZVRT features of the wind turbines maintain the generators coupled to the utility grid. In addition, natural transients, e.g., wind gusts may also induce a spike in energy generation.
Under such transient conditions, the power converter dissipates at least some of the stored energy therein as well as the energy still being generated by the generator that has not been removed from service. Some known dissipative circuits, i.e., dynamic breaking circuits, associated with power converters include at least one fast switching device, e.g., an insulated gate bipolar transistor (IGBT), a gate turn-off thyristor (GTO), or a silicon-controlled rectifier (SCR), in series with a resistive device. In the event of a voltage surge on the DC buses of the power converter due to increased stored energy, the switching devices of the dissipative circuit will open and close to transmit DC current to the resistive device, wherein the electric current is dissipated as heat energy. These extra components increase the size, weight, and cost of power converters.